Metal stripping additive, composition containing the same, and method for stripping metal by using the composition

ABSTRACT

The present invention provides a metal stripping additive, composition containing the same, and method for stripping metal by using the composition. The metal stripping additive comprises a phosphate, a carbonate, and a component selected from at least one of citric acid or a derivative thereof, oxalate or a derivative thereof, malate or a derivative thereof. The metal stripping additive is used with nitric acid as the metal stripping composition of the present invention. The present method has advantages of being capable of stripping various metals, low corrosion, low toxicity, and being applicable under ambient temperature.

RELATED APPLICATIONS

The application claims priority to Taiwan Application Serial Number TW102106178, filed on Feb. 22, 2013, which is herein incorporated byreference.

BACKGROUND

1. Technical Field

The present invention is related to a metal stripping additive andcomposition containing the same; more particularly to a gold strippingadditive and composition containing the same.

2. Description of Related Art

From ancient times, minerals are closely linked with the development ofhuman society and science. Nowadays, their importance is even more inthe electronic industry of modern technology. Various elements ofelectronic industry, such as a central processing unit, a printedcircuit board, a memory, a filp chip, or even a concealed antennaequipped inside the outer shell of a mobile phone, contain metal-platedparts. In fact, those metal-plated parts play important roles in thedevice. However, minerals resources are not unlimited; therefore, underthe sustainable development consideration, how to recycle the metalmaterials from those metal-plated parts has become a critical task inthe industry.

Unlike most of the other metals, gold is an inert metal with highlystable chemical properties, therefore, it is not easy to strip it fromwaste products and there is only a few recycling methods are available.Aqua Regia method and cyanide method are the two gold-stripping methodsused in the field.

Aqua regia is an acid liquid made by mixing concentrated nitric acid andconcentrated hydrochloric acid at a volume ratio of 1:3. It is capableof dissolving most of the metal materials including gold. Aqua regia hasextreme corrosion; therefore, even if the preparation thereof is easy,it still is not the dominant choice in the field under safety concern.Moreover, except for the metal-plated parts desired to be recycled, thesubstrate itself or other components of the electronic elements are alsovaluable. That said, it is not ideal because if aqua regia is used forrecycling the metal material from waste electronic elements, the othercomponents of the waste electronic elements will also be destroyed bythe extreme corrosion property of aqua regia and fail to be recycled.

Cyanide method is the mostly used gold-stripping method in the field.Although cyanide does not have the disadvantage of harmful corrosionproperty as aqua regia does, it has high bio-toxicity that causes highrisk in operation and storage. In addition, the waste liquid resultingfrom cyanide method contains a high concentration of cyanide. In orderto comply with the environmental safety regulations, the waste liquidhas to be diluted for at least 500 fold. This additional processing costis also a burden for business.

Taiwanese Patent Publication No. 201247941 discloses a novelgold-stripping composition. Its components while operating withelectrolysis method effectively strip gold from recycle materials.However, the application of the aforesaid gold-stripping composition islimited as some electronic elements adopt substrates with noconductivity.

In light of the foregoing, a metal-stripping composition beingoperating-friendly and able to be widely applied is in need in thefield.

SUMMARY

One object of the present invention is to provide a metal-strippingadditive, which has low corrosion, low toxicity and neutral pH value andis favorable for metal-stripping operation.

Another object of the present invention is to provide a metal-strippingcomposition, which is operating-friendly and is operable under roomtemperature.

More another object of the present invention is to provide ametal-stripping method, which is widely suitable for various kinds ofmetal and has better stripping rate.

In order to achieve the above objects, the present invention provides ametal stripping additive, comprising: 10 to 40 wt % of phosphate; 3 to15 wt % of carbonate; and 5 to 15 wt % of a component selected from atleast one of citric acid or a derivative thereof, oxalate or aderivative thereof, or malate or a derivative thereof.

Preferably, the metal stripping additive further comprises 5 to 30 wt %of a component selected from at least one of sodium hypochlorite or aderivative thereof, or pyridinesulfonic acid or a derivative thereof.

Preferably, said carbonate is sodium bicarbonate, potassium bicarbonate,sodium carbonate, or a combination thereof.

Preferably, said phosphate is monobasic potassium phosphate, dipotassiumhydrogen phosphate, or a combination thereof.

Preferably, said citric acid or a derivative thereof is citric acidmonohydrate, trisodium citrate monohydrate, or a combination thereof.

Preferably, the metal stripping additive comprises a component selectedfrom at least one of boric acid, sulfate, metal chloride, nitrate, metalhydroxide.

Preferably, said sulfate is sodium sulfate, ammonium sulfate, potassiumsulfate, or a combination thereof.

Preferably, said metal chloride is sodium chloride, ammonium chloride,potassium chloride, or a combination thereof.

Preferably, said nitrate is ammonium nitrate, potassium nitrate, sodiumnitrate, or a combination thereof.

Preferably, said metal hydroxide is sodium hydroxide, potassiumhydroxide, or a combination thereof.

Preferably, the metal stripping additive has a pH value of 6.5 to 8.0.

The present invention also provides a metal stripping composition,comprising: 40 to 60% (v/v) of nitric acid; and 40 to 60% (v/v) of themetal stripping additive.

Preferably, said nitric acid has a concentration of 60 to 70 wt %.

The present invention also provides a method for stripping metal,comprising the following steps: contacting an object to be processedwith the metal stripping composition.

Preferably, said contacting is accomplished by immersing said object tobe processed into said metal stripping composition.

Preferably, said contacting is accomplished at a temperature of 10° C.to 70° C.

Preferably, said contacting is accomplished in a time period of 10 secto 900 sec.

Preferably, the method has a stripping rate of 0.01 μm/min to 0.2μm/min.

Preferably, said object to be processed is an electronic element.Preferably, said electronic element is a central processing unit, aprinted circuit board, a memory, a filp chip, a mobile phone, acombination thereof.

Preferably, said metal stripping additive and/or said metal strippingcomposition does not have cyanide, lead, cadmium, mercury, chromium(VI),or a combination thereof.

Preferably, said metal is gold, copper, nickel, tin, palladium, or acombination thereof.

Preferably, said metal stripping additive, said metal strippingcomposition, and/or said method has no harm to an object that made ofsilver, stainless steel, titanium, plastic, or a combination thereof.

To sum up, the present invention relates to a metal stripping additivefor stripping various metals, a composition containing the same, and amethod of using the composition. The additive of the present inventionhas neutral pH; therefore, is safer than the conventional methods instorage and operation. In addition, the method of the present inventioncan be operated under room temperature, and the time required is shorterthan the conventional methods. All above features make the presentinvention more superior than the conventional methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a photo of a CPU before-treated recited in theexperimental 1 of embodiment 2 of the present invention.

FIG. 1B shows a photo of a CPU after-treated recited in the experimental1 of embodiment 2 of the present invention.

FIG. 2A shows a photo of a PCB before-treated recited in theexperimental 2 of embodiment 2 of the present invention.

FIG. 2B shows a photo of a PCB after-treated recited in the experimental2 of embodiment 2 of the present invention.

FIG. 3A shows a photo of a PCB before-treated recited in theexperimental 3 of embodiment 2 of the present invention.

FIG. 3B shows a photo of a PCB after-treated recited in the experimental3 of embodiment 2 of the present invention.

FIG. 4A shows a photo of a memory before-treated recited in theexperimental 4 of embodiment 2 of the present invention.

FIG. 4B shows a photo of a memory after-treated recited in theexperimental 4 of embodiment 2 of the present invention.

FIG. 5A shows a photo of a chip before-treated recited in theexperimental 5 of embodiment 2 of the present invention.

FIG. 5B shows a photo of a chip after-treated recited in theexperimental 5 of embodiment 2 of the present invention.

FIG. 6A shows a photo of a chip before-treated recited in theexperimental 6 of embodiment 2 of the present invention.

FIG. 6B shows a photo of a chip after-treated recited in theexperimental 6 of embodiment 2 of the present invention.

FIG. 7A shows a photo of an outer shell of a mobile phone before-treatedrecited in the experimental 7 of embodiment 2 of the present invention.

FIG. 7B shows a photo of an outer shell of a mobile phone after-treatedrecited in the experimental 7 of embodiment 2 of the present invention.

FIG. 8A shows a photo of a stainless steel before-treated recited in theexperimental 8 of embodiment 2 of the present invention.

FIG. 8B shows a photo of a stainless steel after-treated recited in theexperimental 8 of embodiment 2 of the present invention.

DETAILED DESCRIPTION

In appreciate the insufficiency of the conventional metal strippingmethods, the present invention provides a metal stripping additive and acomposition containing the same. The present metal stripping compositionis not only suitable for stripping gold but also suitable for strippingmaterials such as copper, nickel, tin, palladium; therefore is favorablefor widely metal recycle process.

The metal stripping additive of the present invention comprises: 10 to40 wt % of phosphate; 3 to 15 wt % of carbonate; and 5 to 15 wt % of acomponent selected from at least one of citric acid or a derivativethereof, oxalate or a derivative thereof, or malate or a derivativethereof. It is surprises that the aforesaid components while mixing atthe aforesaid relative concentration results a metal stripping additivedisplaying superior metal stripping effect with nitric acid.Furthermore, the present metal stripping additive has a pH value whichis at the neutral range of 6.5 to 8.0; therefore is safe in both ofstorage and operation.

In an alternative embodiment of the present invention, said carbonate issodium bicarbonate, potassium bicarbonate, sodium carbonate, or acombination thereof; said phosphate is monobasic potassium phosphate,dipotassium hydrogen phosphate, or a combination thereof; said citricacid or a derivative thereof is citric acid monohydrate, trisodiumcitrate monohydrate, or a combination thereof.

In an alternative embodiment of the present invention, the presentadditive further comprises 5 to 30 wt % of a component selected from atleast one of sodium hypochlorite or a derivative thereof, orpyridinesulfonic acid or a derivative thereof.

In a preferable embodiment of the present invention, the presentadditive comprises 10 to 40 wt % of monobasic potassium phosphate; 5 to30 wt % of sodium hypochlorite; 3 to 15 wt % of sodium bicarbonate; and5 to 15 wt % of citric acid monohydrate.

In an alternative embodiment of the present invention, the metalstripping additive may further comprise a component selected from atleast one of boric acid, sulfate, metal chloride, nitrate, metalhydroxide. Alternatively, said sulfate is sodium sulfate, ammoniumsulfate, potassium sulfate, or a combination thereof; said metalchloride is sodium chloride, ammonium chloride, potassium chloride, or acombination thereof. Said metal chloride is sodium chloride, ammoniumchloride, potassium chloride, or a combination thereof. Said nitrate isammonium nitrate, potassium nitrate, sodium nitrate, or a combinationthereof. Said metal hydroxide is sodium hydroxide, potassium hydroxide,or a combination thereof.

The metal stripping composition of the present invention is formulatedby the metal stripping additive and nitric acid. More specifically, themetal stripping composition of the present invention comprises: 40 to60% (v/v) of nitric acid; and 40 to 60% (v/v) of the metal strippingadditive. Alternatively, said nitric acid has a concentration of 60 to70 wt %; preferably, said nitric acid is a commercial nitric acid havinga concentration of 68 wt %.

In the preferable embodiment of the present invention, both of the metalstripping additive and the metal stripping composition does not havecyanide, lead, cadmium, mercury, chromium(VI) or a combination thereof.Therefore, they have no harm to the environment and to the health of theuser while operation.

One of the features of the present metal stripping additive and/or themetal stripping composition is that it (or they) is useful in resolvingor stripping the gold, copper, nickel, tin, palladium or a combinationthereof on the object to be processed.

One of the features of the present metal stripping additive and/or themetal stripping composition is that it (or they) has no harm to theobject made of silver, stainless steel, titanium, plastic, or acombination thereof. As used herewith, the term “no harm” means thepresent metal stripping additive and/or the metal stripping compositionsubstantially do not dissolve or strip the object made of the aforesaidmaterial.

One of the features of the present metal stripping additive and/or themetal stripping composition is that it (or they) contains no componentthat is adverse to organisms or the environment, such as, cyanide, lead,cadmium, mercury, or chromium(VI).

The present invention also provides a method for stripping metal,comprising the following steps: contacting an object to be processedwith the metal stripping composition. More specifically, said object tobe processed is, but not limited to central processing unit, printedcircuit board, memory, flip chip, mobile phone, or a combinationthereof.

Said contacting is accomplished by immersing said object to be processedinto said metal stripping composition; more specifically, by immersingsaid object to be processed into a bath of said metal strippingcomposition in a container and letting the area containing the desiredmetal completely being immersed beneath said metal strippingcomposition.

By the disclosure of the present invention, those having ordinary skillin the art to which the present invention pertains can adjust thecontacting time and temperature based on its practice. Preferably, thepresent method can be operated at a temperature of 10 to 70° C. Althoughhigher temperature gives more rapid reaction rate, the present inventionobtains good metal stripping efficiency under room temperature;therefore the present invention has the advantage that no heating isrequired.

Moreover, depending on the amount of the metal to be recycled, thosehaving ordinary skill in the art can adjust the contacting time of thepresent invention according to the disclosure of the present invention.Taking gold as an example, the stripping rate of the present method is0.01 to 0.2 μm/min. Generally, it only takes 10 to 900 sec. forcontacting to accomplish the metal stripping process of the presentmethod.

The metal stripping composition after processed dissolves the strippedmetal, and the metal can be purified and isolated for other applicationsby the known manners (, that is, purification methods such aselectrolysis, ion-exchange, etc.) in the field.

The following examples are used to demonstrate the experiments conductedin the research of the present invention for further displaying thefeatures and advantages of the present invention. It shall be understoodthat the following embodiments are only exemplary and shall not limitthe claimed scope of the present invention.

Embodiment 1 Test for Stripping Saturation of the Present StrippingComposition to Various Kinds of Metals

The stripping additive according to the present invention used in thisembodiment contained the following components: 40 weight percentage ofmonobasic potassium phosphate, 30 weight percentage of sodiumhypochlorite, 15 weight percentage of sodium bicarbonate, and 15 weightpercentage of citric acid monohydrate. The pH thereof was 7.4. Theformulated stripping additive was mixed with a commercial nitric acid(68 wt %) by a volume ratio of 5:5 to obtain the metal strippingcomposition of the present invention.

Pure materials of various weight were respectively put into said metalstripping composition to test the solubility of different materials insaid metal stripping composition. The materials tested in thisembodiment were gold, copper, nickel, tin, silver, stainless steel,titanium, and plastic. The operation temperature was 30° C.

The maximum solubility of the present metal stripping composition toeach material, that is, the stripping saturation desired to know, wasobtained from the solubility of each material of different weight. Theexperimental result is listed in the following table 1.

TABLE 1 the maximum solubility (stripping saturation) of the metalstripping composition of the embodiment to various materials materialstripping saturation (g/L) gold 13.5 copper 140 nickel 170 tin 160silver not dissolved stainless steel not dissolved titanium notdissolved plastic not dissolved

According to the experimental results, the stripping saturation of thepresent metal stripping composition to gold, copper, nickel and tin wasobtained, and it was noted that the present metal stripping compositionhas no solubility to silver, stainless steel, titanium, and plastic.That said, for example, if the present metal stripping composition isused to strip the gold layer plated on a plastic substrate, the presentmetal stripping composition has no harm to the plastic substrate.

Embodiment 2 Testing the Effect of the Present Stripping Composition byUsing Recycle Materials

The tests in this embodiment were conducted by the metal strippingcomposition formulated in the embodiment 1.

Experiment 1

A CPU plated with a gold layer of 1.25 μm in thickness was obtained andimmersed in the metal stripping composition of this embodiment. Thetreatment was continued at 30° C. for 12 minutes. FIG. 1A displayed theCPU before treated, and FIG. 1B displayed that after treated. Accordingto the results shown in the figures, the present metal strippingcomposition had completely stripped the plated gold layer and had noharm to the CPU substrate.

Experiment 2

A PCB plated with a gold layer and a copper layer (both of them were0.075 μm in thickness) was obtained and immersed in the metal strippingcomposition of this embodiment. The treatment was continued at 30° C.for 3 minutes. FIG. 2A displayed the PCB before treated, and FIG. 2Bdisplayed that after treated. According to the results shown in thefigures, the present metal stripping composition had completely strippedthe plated gold layer and copper layer and had no harm to the PCBsubstrate.

Experiment 3

A PCB plated with a gold layer was obtained and immersed in the metalstripping composition of this embodiment. The treatment was continued at30° C. for 7 minutes. FIG. 3A displayed the PCB before treated, and FIG.3B displayed that after treated. According to the results shown in thefigures, the present metal stripping composition had completely strippedthe plated gold layer and had no harm to the PCB substrate.

Experiment 4

A memory plated with a gold layer (gold fingers) was obtained andimmersed in the metal stripping composition of this embodiment. Thetreatment was continued at 30° C. for 3 minutes. FIG. 4A displayed thememory before treated, and FIG. 4B displayed that after treated.According to the results shown in the figures, the present metalstripping composition had completely stripped the plated gold layer andhad no harm to the substrate of the memory.

Experiment 5

A flip chip plated with a gold layer (gold fingers) was obtained andimmersed in the metal stripping composition of this embodiment. Thetreatment was continued at 30° C. for 2 minutes. FIG. 5A displayed theflip chip before treated, and FIG. 5B displayed that after treated.According to the results shown in the figures, the present metalstripping composition had completely stripped the plated gold layer andhad no harm to the substrate of the flip chip.

Experiment 6

A flip chip plated with a gold layer (gold fingers) was obtained andimmersed in the metal stripping composition of this embodiment. Thetreatment was continued at 30° C. for 9 minutes. FIG. 6A displayed theflip chip before treated, and FIG. 6B displayed that after treated.According to the results shown in the figures, the present metalstripping composition had completely stripped the plated gold layer andhad no harm to the substrate of the flip chip.

Experiment 7

An outer shell of a mobile phone plated with a gold layer as a concealedantenna was obtained and immersed in the metal stripping composition ofthis embodiment. The treatment was continued at 30° C. for 90 sec. FIG.7A displayed the shell before treated, and FIG. 7B displayed that aftertreated. According to the results shown in the figures, the presentmetal stripping composition had completely stripped the plated goldlayer on the mobile phone shell and had no harm to the shell.

Experiment 8

A stainless steel plate plated with a gold layer was obtained andimmersed in the metal stripping composition of this embodiment. Thetreatment was continued at 30° C. for 60 sec. FIG. 8A displayed theplate before treated, and FIG. 8B displayed that after treated.According to the results shown in the figures, the present metalstripping composition had completely stripped the plated gold layer onthe plate and had no harm to it.

The aforementioned experiments 1-8 showed that the present metalstripping composition is able to rapidly strip the gold or coppermaterials on an electronic element at room temperature and has no harmto other materials on the electronic element concerned. That said,except for the strippable materials on the electronic element, otherrecycling process can be conducted for other materials and the recyclevalue of waste products can be maximized.

Embodiment 3 Testing the Effect of the Present Stripping Composition atVarious Operation Temperatures

The tests in this embodiment were conducted by the metal strippingcomposition formulated in the embodiment 1.

Three PCBs plated with a gold layer of 0.075 μm in thickness wereobtained and immersed in the metal stripping composition of thisembodiment. The treatment was conducted at 10° C., 30° C., and 50° C.,respectively. The time for completely stripping the gold layer wasrecorded and calculated into stripping rate defined as μm/min. Theexperimental results were listed in the following table 2.

TABLE 2 The stripping rate of the present metal stripping compositionfor gold layers at different operation temperatures. Temp. strippingrate (μm/min) 10° C. 0.025 30° C. 0.05 50° C. 0.075

From the results recorded in the table, it was noted that the strippingrate of the present metal stripping composition was increased along withthe raising temperature. If the user considers saving the heating cost,the present composition showed superior stripping rate at roomtemperature (30° C.). Furthermore, the present composition showed goodstripping rate even at low operation temperature (10° C.), which meansthe present composition can still has good working efficiency at highlatitude area.

Embodiment 4 Comparing the Efficiency Between the Metal StrippingComposition of the Present Invention and Conventional Methods

The tests in this embodiment were conducted by the metal strippingcomposition formulated in the embodiment 1. Aqua regia and cyanideconventional in the field were also used as comparison.

Three PCBs plated with a gold layer of 0.075 μm in thickness wereobtained and immersed respectively in the metal stripping composition ofthis embodiment, aqua regia, and cyanide. As the conventional method wasrequired to be operated at 50□, the experiment of this embodiment wasconducted at 50° C. The time for completely stripping the gold layer wasrecorded and calculated into stripping rate defined as μm/min. Theexperimental results were listed in the following table 3.

TABLE 3 Comparing the efficiency between the metal stripping compositionof the present invention and conventional methods. Stripping methodstripping rate (μm/min) The present metal stripping composition 0.05aqua regia 0.005 cyanide 0.025

According to the aforesaid experimental results, the metal strippingcomposition of the present invention not only has no drawbacks of highcorrosion and high toxicity, but also displayed stripping rate that farbetter than the conventional methods. It was obvious that the presentcomposition is a better choice than conventional methods no matter insafety or working efficiency.

Those having ordinary skill in the art can understand variousmodifications according to the disclosed embodiments without departingfrom the spirit of the present invention. Therefore, the above-recitedembodiments shall not be used to limit the present invention but shallintend to cover all modifications under the spirit and scope of thepresent invention along with the attached claims.

What is claimed is:
 1. A metal stripping additive, comprising: 10 to 40wt % of phosphate; 3 to 15 wt % of carbonate; and 5 to 15 wt % of acomponent selected from at least one of citric acid or a derivativethereof, oxalate or a derivative thereof, or malate or a derivativethereof.
 2. The metal stripping additive according to claim 1, furthercomprising 5 to 30 wt % of a component selected from at least one ofsodium hypochlorite or a derivative thereof, or pyridinesulfonic acid ora derivative thereof.
 3. The metal stripping additive according to claim1, wherein said carbonate is sodium bicarbonate, potassium bicarbonate,sodium carbonate, or a combination thereof.
 4. The metal strippingadditive according to claim 1, wherein said phosphate is monobasicpotassium phosphate, dipotassium hydrogen phosphate, or a combinationthereof.
 5. The metal stripping additive according to claim 1, whereinsaid citric acid or a derivative thereof is citric acid monohydrate,trisodium citrate monohydrate, or a combination thereof.
 6. The metalstripping additive according to claim 1, further comprising a componentselected from at least one of boric acid, sulfate, metal chloride,nitrate, or metal hydroxide.
 7. The metal stripping additive accordingto claim 6, wherein said sulfate is sodium sulfate, ammonium sulfate,potassium sulfate, or a combination thereof.
 8. The metal strippingadditive according to claim 6, wherein said metal chloride is sodiumchloride, ammonium chloride, potassium chloride, or a combinationthereof.
 9. The metal stripping additive according to claim 6, whereinsaid nitrate is ammonium nitrate, potassium nitrate, sodium nitrate, ora combination thereof.
 10. The metal stripping additive according toclaim 6, wherein said metal hydroxide is sodium hydroxide, potassiumhydroxide, or a combination thereof.
 11. The metal stripping additiveaccording to claim 1, having a pH value of 6.5 to 8.0.
 12. The metalstripping additive according to claim 1, wherein said metal strippingadditive does not have cyanide, lead, cadmium, mercury, chromium(VI), ora combination thereof.
 13. The metal stripping additive according toclaim 1, wherein said metal is gold, copper, nickel, tin, palladium, ora combination thereof.
 14. The metal stripping additive according toclaim 1, having no harm to an object that made of silver, stainlesssteel, titanium, plastic, or a combination thereof.
 15. A metalstripping composition, comprising: 40 to 60% (v/v) of nitric acid; and40 to 60% (v/v) of the metal stripping additive according to claim 1.16. The metal stripping composition according to claim 15, wherein saidnitric acid has a concentration of 60 to 70 wt %.
 17. The metalstripping composition according to claim 15, wherein said metalstripping composition does not have cyanide, lead, cadmium, mercury,chromium(VI), or a combination thereof.
 18. The metal strippingcomposition according to claim 15, wherein said metal is gold, copper,nickel, tin, palladium, or a combination thereof.
 19. The metalstripping composition according to claim 15, having no harm to an objectthat made of silver, stainless steel, titanium, plastic, or acombination thereof.
 20. A method for stripping metal, comprising thefollowing steps: contacting an object to be processed with the metalstripping composition according to claim
 15. 21. The method according toclaim 20, wherein said contacting is accomplished by immersing saidobject to be processed into said metal stripping composition.
 22. Themethod according to claim 20, wherein said contacting is accomplished ata temperature of 10° C. to 70° C.
 23. The method according to claim 20,wherein said contacting is accomplished in a time period of 10 sec to900 sec.
 24. The method according to claim 20, having a stripping rateof 0.01 μm/min to 0.2 μm/min.
 25. The method according to claim 20,wherein said metal is gold, copper, nickel, tin, palladium, or acombination thereof.
 26. The method according to claim 20, having noharm to an object that made of silver, stainless steel, titanium,plastic, or a combination thereof.
 27. The method according to claim 20,wherein said object to be processed is an electronic element.
 28. Themethod according to claim 27, wherein said electronic element is acentral processing unit, a printed circuit board, a memory, a filp chip,a mobile phone, a combination thereof.